<!DOCTYPE html>
<head>
  <title>Document</title>
  <style>
    .content {
      width: 1100px;
      margin: 0 auto;
      height: 800px;
      background: #ccc;
    }
    .item {
      width: 221px;
      height: 168px;
      background: orange;
      color: #fff;
      float: left;
      border: 1px solid #000;
      margin-right: -1px; /* 使用负值，使上一个元素右边框覆盖下一个元素的左边框 */
      box-sizing: border-box;
    }
    .item.first {
      width: 220px; /* 给第一个小盒子，宽度减去1px，即保持平分的宽度 220px */
    }
  </style>
</head>
<body>
  <div class="content">
    <div class="box">
      <div class="item first">1</div>
      <div class="item">2</div>
      <div class="item">3</div>
      <div class="item">4</div>
      <div class="item">5</div>
    </div>
  </div>
</body>
</html>